Passive smoking in childhood significantly increased risk of RA in adult smokers. The results of a study were presented at the Annual European Congress of Rheumatology (EULAR) 2017 conference.

To analyse the impact of active and passive smoking on the risk of developing RA, a large population of female volunteers born between 1925 and 1950 were prospectively followed since 1990. Passive smoking exposure during childhood increased the association between RA risk and adult active smoking. In smokers who had childhood passive exposure to smoke, the hazard ratio was 1.73 compared with non–smokers not exposed during childhood. In contrast, the hazard ratio was 1.37 in active smokers not exposed to passive smoke during childhood.

"Our study highlights the importance of avoiding any tobacco environment in children, especially in those with a family history of RA," said lead author Professor Raphaèle Seror from University Hospitals of South Paris, France.

In a separate analysis looking for a potential association between the development of RA and a history of disrupted bowel function, previous chronic diarrhoea was associated with more than double the risk of RA (hazard ratio of 2.32), while chronic constipation or alternating between diarrhoea and constipation did not impact risk (hazard ratios of 1.16 and 1.07 respectively).

"An association between a history of chronic diarrhoea and the risk of developing RA supports the hypothesis of dysbiosis (a bacterial imbalance in the gut) as a risk factor for the emergence of immune–mediated inflammatory disease," explained Professor Seror. These data fit perfectly with the preclinical scheme of RA where an external event occurs at an early stage to promote emergence of so–called auto–immunity, followed years later by clinical RA," she concluded.

Out of 70,598 women, a total of 1,239 patients self–reported developing RA, of which 350 cases were eligible for analysis of the link to active and passive smoking, and 280 in the analysis of the link to a history of an intestinal transit disorder. The mean age at inclusion in the study was 49.0 years; the mean duration of follow–up was 21.2 years.

11 self–administered questionnaires were sent to the participants between 1990 and 2014 to collect medical, demographic, environmental and hormonal data and dietary habits. The diagnosis of RA was collected on 2 successive questionnaires. Cases were considered certain if, having declared RA, they had taken a RA specific medication (methotrexate, leflunomide or a biologic) since 2004 (the period from which drug reimbursement data was available). Women were excluded if they had an inflammatory bowel disease and / or no information on their smoking status.

Passive smoking was assessed by the following question: "When you were children, did you stay in a smoky room?" Patients were considered exposed if the answer was "yes, a few hours, or yes, several hours a day". The usual intestinal transit, reported by women prior to RA diagnosis (on average 10 years), were classified as normal transit, chronic diarrhoea, chronic constipation, and alternating between diarrhoea and constipation.